Croatians are voting in a referendum that could ban same-sex marriage in this conservative and Catholic country, an issue that polarises the European Union's newest member.

Passions have run high in Croatia during the past few weeks as opponents and supporters of the vote had heated debates over gay marriage.

The referendum on Sunday on whether to amend the country's constitution to define marriage as a "union between a woman and a man" is the result of a Catholic Church-backed initiative. Croatia's constitution currently does not define marriage.

Polling stations are open and first partial results are expected late on Sunday.

The centre-left government's announcement of a bill enabling gay couples to register as "life partners" sparked fears among many conservatives in Croatia - which only joined the European Union in July - that same-sex marriage would be allowed.

In May, the In the Name of the Family group, backed by the Church, collected almost 700,000 signatures seeking a nationwide vote on a definition of marriage.

"We believe that marriage, children and family are such important issues that the whole society has to decide on them," the leader of the initiative, Zeljka Markic, told AFP.

But the government, human rights activists and prominent public figures have all spoken out against the referendum, urging people to cast a 'no' vote.

"It would not be good for Croatia to appear to be a country of intolerance," President Ivo Josipovic told journalists.

The latest survey showed that 68 per cent of Croatians plan to vote 'yes' on Sunday compared to 27 per cent against.

"We are a Catholic country and the family is very important in today's world. Why destroy that which exists for centuries?" Jasna Badovinac, a 62-year-old pensioner told AFP after she voted 'yes' at a Zagreb polling station on Sunday.

On Saturday more than 1,000 people braved the cold and rainy weather to take part in a protest march through central Zagreb calling the referendum discriminatory.

Carrying banners reading "Against fascism" or "Homosexuality is not a choice but hatred is", they also warned that the referendum might lead to other conservative initiatives targeting minorities or sensitive issues such as abortion.

"How would you feel if you had to ask four million people for permission to get married?" said Ljubomir Mateljan, a 31-year-old gay rights activist from Split, on the central Adriatic coast, told AFP.

But in a country where almost 90 per cent of the population are Roman Catholics, the still powerful Church has called on followers to vote 'yes'.

"Marriage is the only union enabling procreation," said Croatia's Cardinal Josip Bozanic in a letter read out in churches.